The Intergranular Corrosion Behavior Of 6000-series Alloys With Different Mg/Si And Cu Content

Addition of Cu or excess Si in 6000-series aluminium alloys can improve the strength of alloys.But 6000-series aluminium alloys with high Cu or excess Si addition were susceptible to intergranular corrosion(IGC).In order to obtain good IGC resistance without strength loss,four alloys with low/high Cu and various Mg/Si ratio were designed.The corrosion behaviour of four alloys was investigated by accelerated corrosion test and electrochemical test.Microstructures were observed by SEM and TEM to analyze the IGC mechanism.Comparing the IGC resistance and mechanical properties to obtain optimizing alloying strategy.The main results are shown as below:(1)The peak aged alloy B was attacked by pitting,and alloy A,C and D were susceptible to IGC.The IGC resistance was sorted in: B>A?D>C.The comprehensive mechanical properties were sorted in: D>C>A>B.(2)Alloy D possessed the best mechanical property and its IGC resistance was comparable to the commercial alloy A(6022).So alloy D was the alloy with best comprehensive property.The optimizing alloying strategy was Mg/Si=2.05 and Cu=0.5wt.%,which proved that good IGC resistance can be obtained by reasonable Mg/Si ratio in high Cu alloy,which provides possibility for developing new alloy.(3)It was revealed that IGC susceptibility of alloys was the result of microgalvanic coupling between the noble grain boundary precipitates(GBPts)and the adjacent precipitates free zone(PFZ): The GBPts of alloy A and B were ?' phase,the GBPts of alloy C and D were Q' phase.(4)Cu and excess Si made alloys be susceptible to IGC.Because Q' phase precipitated on GB of high Cu alloys acted as an effective cathode and influenced the IGC more than ?' phase.Si particles or AlFeSi(Mn,Cu)phase found in alloys with excess Si formed a continuous corrosion channel with ?' phase or Q' phase,which was bad for IGC resistance.(5)The interaction of Cu and Mg/Si ratio affected the type,size and distribution of GBPts to determine the alloy property.Alloy D possed better comprehensive property than alloy A,because Cu can refine the microstructure and promote to precipitate Q' phase and L phase,which was benefit to mechanical property and made alloys susceptible to IGC.But in alloy D,fine Q' can't form continuous corrosion channel,which help to reduce the IGC susceptibility.